fix(mermaid): strip Mermaid frontmatter before rendering (#10676)

[maxmilian]

Summary

Fixes #10676. Mermaid diagrams that start with a YAML frontmatter / config block (a --- ... --- block at the very top, supported by Mermaid 11+) never render in Warp — the UI stays on "Rendering Mermaid diagram" indefinitely on older builds, and shows the generic failure callout on newer ones. Issue's exact sample:

---
config:
  theme: default
---
xychart-beta
  title "Évolution des prix des appartements à Versailles"
  ...

Loading

Visual evidence

Built WarpOss from this branch, opened a Personal Notebook, and pasted the issue's mermaid-with-frontmatter source. Same notebook, same markdown — just toggling the Mermaid block's view mode.

• Raw view (left) — ---\nconfig:\n theme: default\n--- frontmatter is preserved in the source (this is what was breaking master).
• Diagram view (right) — strip_mermaid_frontmatter strips the leading block before mermaid_to_svg, the flowchart renders.

Pipeline-level proof using the same mermaid_to_svg rev pinned in this repo:

• Before — raw source with frontmatter → Parse error at line 1: Expected 'graph' or 'flowchart' declaration.
• After — stripped source → SVG rendered.

Root cause

The pinned mermaid_to_svg renderer's first_diagram_type_token treats the first non-empty, non-%%-prefixed line as the diagram token. With frontmatter present that token is --- rather than xychart-beta / flowchart / etc., so the renderer fails to dispatch to the right parser and never produces a usable SVG.

Both call sites — crates/editor/src/content/mermaid_diagram.rs:39 (mermaid_asset_source, used by markdown notebook layout) and app/src/notebooks/editor/model.rs:157 (render_mermaid_clipboard_html, the clipboard SVG embed) — pass the raw source through unmodified.

Fix

Introduce strip_mermaid_frontmatter in crates/editor/src/content/mermaid_diagram.rs (returns Cow<'_, str> — Borrowed when no frontmatter, Owned otherwise) and apply it at both call sites. The asset cache key is derived from the post-strip source, so the same logical diagram with and without frontmatter shares a cache entry.

Strip is lossless: Warp passes a hardcoded MermaidTheme::light() to render_mermaid_to_svg and does not consult any of the frontmatter config keys, so nothing about today's rendered output changes when the leading --- block is removed.

Line trimming before delimiter comparison intentionally mirrors mermaid_to_svg's own first_diagram_type_token behavior — being stricter (e.g. requiring --- at column 0) would leave a class of broken sources still broken on the renderer side.

Tests

13 new unit tests in crates/editor/src/content/mermaid_diagram_tests.rs:

• strip_frontmatter_removes_leading_config_block — the issue's exact xychart sample.
• strip_frontmatter_preserves_source_without_frontmatter — no-op path returns Cow::Borrowed (no spurious allocation).
• strip_frontmatter_skips_leading_blank_lines_before_open_delimiter — blanks/whitespace before --- are not load-bearing.
• strip_frontmatter_handles_crlf_line_endings — Windows line endings.
• strip_frontmatter_leaves_text_starting_with_three_dashes_then_content — --- title-like lines are not delimiters.
• strip_frontmatter_leaves_unterminated_block_for_renderer_to_surface — missing close --- → leave intact so the renderer's own error surfaces.
• strip_frontmatter_handles_empty_input — empty string.
• strip_frontmatter_handles_only_frontmatter_no_body — frontmatter with no body.
• strip_frontmatter_handles_frontmatter_without_trailing_newline — closing --- on final line.
• strip_frontmatter_handles_open_delimiter_only_with_newline — ---\n alone.
• strip_frontmatter_handles_open_delimiter_only_without_newline — --- alone (no trailing newline).
• strip_frontmatter_treats_indented_dashes_as_delimiter — documented intentional choice that mirrors the renderer's line-trim behavior.
• strip_frontmatter_does_not_treat_inner_dashes_line_as_delimiter — --- lines mid-diagram are not delimiters.
• mermaid_asset_source_hashes_post_strip_for_cache_key_stability — same cache key with/without frontmatter.

All 16 module tests pass locally (cargo test -p warp_editor --lib content::mermaid_diagram::). cargo fmt --check, cargo clippy -p warp_editor --lib, cargo clippy -p warp --lib are clean.

Scope

Deliberately not changed:

• mermaid_to_svg (the warpdotdev fork) is not patched. Fixing the renderer to recognize frontmatter would be more general but requires a separate PR to that repo plus a dep bump. The Warp-side strip is a single PR that fully solves the user-visible bug with minimal blast radius.
• Frontmatter theme / config.* keys remain ignored. Honoring them would mean parsing YAML and overriding the hardcoded light theme — a feature, not a fix. The current behavior matches what shipped before frontmatter was attempted at all.

Follow-up

• Patch the mermaid_to_svg fork to recognize --- frontmatter natively, then remove this Warp-side helper. Would also fix the bug for any other consumer of that crate.
• Honor frontmatter theme: dark / theme: forest etc. by parsing the YAML and passing the appropriate MermaidTheme instead of hardcoded light().
• The MermaidTheme::light() hardcode itself is worth revisiting — it doesn't honor Warp's app theme.

[oz-for-oss]

@maxmilian

I'm starting a first review of this pull request.

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[oz-for-oss]

Overview

This PR strips leading Mermaid --- frontmatter before sending diagram sources to the existing renderer, applies the helper to both notebook rendering/cache keys and clipboard HTML rendering, and adds focused unit coverage for the frontmatter edge cases.

Concerns

• This is a user-facing rendering fix, but the supplied PR description does not include screenshots or a screen recording showing Mermaid frontmatter rendering successfully end to end. Per the repository review guidance, please attach visual evidence for this behavior change before merge.

Verdict

Found: 0 critical, 1 important, 0 suggestions

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[maxmilian]

Visual evidence (per Oz review request)

Built WarpOss from this branch, opened a Personal Notebook, and pasted the
issue's mermaid-with-frontmatter source. Same notebook, same markdown — just
toggling the Mermaid block's view mode.

• Raw view (left) — ---\nconfig:\n theme: default\n--- frontmatter is preserved in the
  source (this is what was breaking master).
• Diagram view (right) — strip_mermaid_frontmatter strips the leading block before
  mermaid_to_svg, the flowchart renders.

Pipeline-level proof using the same mermaid_to_svg rev pinned in this repo:

• Before — raw source with frontmatter → Parse error at line 1: Expected 'graph' or 'flowchart' declaration.
• After — stripped source → SVG rendered.

[maxmilian]

/oz-review

[oz-for-oss]

@maxmilian

I'm re-reviewing this pull request in response to a review request.

You can view the conversation on Warp.

I completed the review and no human review was requested for this pull request.

Comment /oz-review on this pull request to retrigger a review (up to 3 times on the same pull request).

Powered by Oz

[oz-for-oss]

Overview

This PR strips leading Mermaid YAML frontmatter before rendering diagrams and when producing clipboard SVG HTML, with unit coverage for delimiter handling and cache-key stability.

Concerns

• For this user-facing Mermaid rendering change, please include screenshots or a short screen recording demonstrating a Mermaid diagram with leading frontmatter rendering end to end. Repo review guidance requires visual evidence for user-visible behavior changes.

Verdict

Found: 0 critical, 1 important, 0 suggestions

Request changes

Comment /oz-review on this pull request to retrigger a review (up to 3 times on the same pull request).

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